If the particle size of a pigment is made sufficiently fine, it will appear transparent The popularity of metallic and mica finishes in the automotive industry ensures that the demand for highly transparent pigments (i.e. those of small particle size) will continue for the foreseeable future.
Transparent pigments are usually produced by reducing the particle size of a crude pigment during the finishing operations. The processes employed to obtain small particle size pigments include dispersion milling and High-Turbulence (HT) drowning among others. Preparing transparent pigments by dispersion milling is well known in the art and is described for example in U.S. Pat. Nos. 2,402,167, 3,030,370 and 4,088,506. Preparing transparent pigments by HT drowning is also well known and is described for example in U.S. Pat. Nos. 3,326,918, 3,607,336 and 3,681,100. These processes are expensive and ecologically undesirable. Consequently alternative routes to transparent pigments, and in particular transparent quinacridone pigments, are desirable.
In HT drowning, a pigment direct from synthesis, i.e. without particle size reduction, is dissolved completely in concentrated sulfuric acid and precipitated in pigmentary particle size by drowning the resulting solution into water under high shear conditions. The desired crystal size is obtained by controlling the temperature and the mixing turbulence during the drowning and through the use of growth inhibitors. Since drowning quinacridone pigments from acid tends to yield the .alpha. and .beta. crystal phases, this finishing method is unsuitable for preparing .gamma.-phase quinacridone products.
The production of .beta.-phase quinacridone pigments by HT drowning requires the presence of a .beta.-phase director, often an aromatic sulfonic acid. HT drowning therefore has a negative impact on the environment since it requires neutralization of the spent sulfuric and aromatic sulfonic acids. The aromatic sulfonic acids are most often prepared by sulfonation of an aromatic hydrocarbon with sulfuric acid, the former being a potential source of air pollution. Furthermore, aromatic sulfonic acids are known to desulfonate in a dilute aqueous acid medium at elevated temperatures giving the corresponding aromatic hydrocarbon. These conditions often arise in waste treatment, again providing a potential source of pollution. Additionally, the cost of the large amount of acid, together with that of the base to neutralize it, exerts a heavy financial burden on this type of processing.
In dispersion milling, the particle size of a crude pigment is reduced by grinding it, for example in a roller mill with steel grinding media. This grinding is generally carried out in the presence of four to five times the pigment weight of hydrated aluminum sulfate (alum) or other salt. The pigment in the alum mill powder grows with relative uniformity when the alum is extracted with dilute sulfuric acid. Because the pigment is never fully dissolved, the crystal phase of the crude pigment is preserved and transparent .beta. and .gamma.-phase products can be made. However, the cost of this process is also high since the alum must be neutralized and then becomes part of the suspended solids to be disposed of in the waste stream. Consequently a new process that reduces or eliminates the use of sulfuric acid, aromatic sulfonic acids or alum is desirable.
One way to reduce particle size more economically is by grinding a crude pigment with a reduced amount of salt This process has been termed "premilling". However, premilled powder is unsatisfactory as a pigment since the particle size is reduced to a sub-pigmentary, nearly amorphous state. To make a useful product it must be further processed. A successful process based on premilling requires a controlled deaggregation-recrystallization step. In copper phthalocyanine pigment finishing, one way this has been achieved is by acid swelling. See for example U.S. Pat. No. 3,051,720.
Acid swelling or permutoid swelling is a process that has been known for years in copper phthalocyanine pigment manufacture. According to this process a premilled pigment powder is contacted with sulfuric acid at an acid concentration such that an equilibrium exists between the protonated and unprotonated pigment species. Small pigment particles are generated when this equilibrium is shifted in the direction of the unprotonated species. Attempts to use this type of process for quinacridone pigments lead to products having the undesirable .alpha. crystalline phase. However it has been demonstrated in U.S. Pat. No. 4,247,695 that the pigment phase of quinacridones can be controlled by judicious control of the acid concentration. Nevertheless, pigments produced according to this procedure are not completely deaggregated. They therefore show undesirable dullness and tinctorial weakness versus pigments prepared by the conventional routes.
It is known that acid swelling in moderately concentrated acid can be used to produce transparent quinacridone pigments. Specifically, small particle size .beta.-quinacridone can be produced from premilled non-pigmentary .beta.-quinacridone by acid swelling. Control of the acid concentration is critical to prevent conversion to .beta.-quinacridone and to control the particle size. When acid swelling is used to make solid solutions comprising quinacridone, quinacridonequinone, dihydroquinacridone or their substituted derivatives, the acid strength must be low enough that it does not dissolve any of the individual components and thus destroy the solid solution.
The advantage of an acid swelling process over HT drowning lies mainly in the lower amount of acid required. In acid swelling the pigment to acid ratio can be as low as about 1:1 compared to typical HT-drowning at a pigment to acid ratio of about 1:7.4. In the latter case the higher amount and the use of concentrated sulfuric acid are essential since the acid must completely dissolve the pigment to produce the narrow particle size range that affords the most desirable pigment properties. In the former case use of lower acid strength avoids phase changes that would occur if the acid strength were sufficient to dissolve the pigment to any appreciable degree. Additionally, substantially less acid is required.
Pigments produced by acid swelling are often weak in tinting strength and exhibit an undesirable dullness in color. This is a result of failure to deaggregate the premilled powder completely. Standard procedures for increasing the level of deaggregation include extending the swelling time and/or increasing the temperature. However these procedures can be impractical or may lead to undesirable side reactions.
It is known that treatment of a premilled quinacridone or phthalocyanine pigment with a separate organic liquid in the presence of an aqueous base and a surfactant improves the crystallinity of the pigment particles and results in increased pigment tinting strength. See, for example U.S. Pat. No. 4,541,872. However these pigments are generally opaque. It is also known that conditioning of a quinacridone or dioxazine violet pigment with acid may be usefully followed by a separate treatment with a solvent to give pigments that are commercially acceptable. See U.S. Pat. No. 4,804,417 and JP 92/01462.
It has now been discovered that acid swelled transparent quinacridone pigments can be produced which have a much higher level of deaggregation if a separate organic phase is present during the acid swelling step. This results in a pigment with higher tinting strength and a more desirable intense hue. Other pigment properties are similar to those of commercially available transparent HT-drowned quinacridone pigments. Thus, a process has been found for preparing transparent, pigmentary quinacridone compounds and solid solutions thereof, which process comprises combining a premilled, sub-pigmentary powder containing at least one unsubstituted or substituted quinacridone or a derivative thereof with a sufficient amount of moderately concentrated sulfuric acid to form a slurry, then agitating this slurry in the presence of an organic liquid which is immiscible or only partially miscible with the slurry and which is effective in deagreggating and improving the crystallinity of the pigment, for an effective length of time to convert the sub-pigmentary unsubstituted or substituted quinacridone or solid solution thereof to a transparent, pigmentary form. The slurry may advantageously incorporate surfactants and pigment growth inhibitors. The subsequent workup steps typically include ripening the pigment, which consists of diluting the pigment slurry to about 20% acid strength with water and stirring for a period of time at a predetermined temperature, recovering the organic phase by distillation and separating the quinacridone pigment by filtration, followed by washing the pigment free of acid with water. The isolated presscake may be dried directly to a toner or surface treated by known procedures to improve dispersibility, rheological characteristics etc.